1. Field of the Invention
This invention relates to a method for the production of highly crystalline olefin polymers.
2. Description of the Prior Art
It is well known that olefins can be polymerized using a Ziegler-Natta catalyst comprising a compound of a transition metal of Groups IV to VI of the periodic table and a metal of Groups I to III of the periodic table or an organo-compound of a metal of Groups I to III of the periodic table. Most commonly, the polymerization is performed at less than about 100.degree. C., and the polymers are obtained in a slurry form.
In this polymerization process, amorphous polymers are formed as by-products in addition to olefin polymers of high stereoregularity which are very valuable industrially.
The amorphous polymer has a low industrial value, and adversely affects the mechanical properties of products formed from the olefin polymers such as films and fibers. Moreover, the formation of the amorphous polymer consumes and thereby wastes the monomeric starting material, and an additional means is required to remove the amorphous polymer. This is a very serious industrial disadvantage.
It can be easily imagined therefore that substantial inhibition of the formation of amorphous polymers would be of great industrial advantage.
Furthermore, in this polymerization process, the catalyst remains as a residue in the resulting olefin polymers, and adversely affects the stability and processability of the polymers. Additional equipment is necessary therefore to remove the residual catalyst and stabilize the polymers.
These disadvantages of the process can be overcome by increasing the activity of the catalyst which is expressed as the yield of olefin polymer per unit weight of catalyst. If the catalytic activity is increased, no equipment for the removal of the residual catalyst is necessary and the manufacturing cost of the olefin polymer can be reduced.
In the production of olefin polymers such as propylene polymers and butene-1 polymers, titanium trichloride is most widely used as the transition metal compound which is a component of the solid catalyst.
The titanium trichloride used for this purpose is obtained by (1) reducing titanium tetrachloride with hydrogen and activating the reduction product by ball-milling, (2) reducing titanium tetrachloride with metallic aluminum, and activating the reduction product by ball-milling [the resulting compound has the general formula TiCl.sub.3 (AlCl.sub.3).sub.1/3 ], or (3) reducing titanium tetrachloride with an organoaluminum compound at about -30.degree. C. to about 10.degree. C. and heating the resulting solid reduction product to about 120.degree. to about 180.degree. C. to change the crystal form of the solid reduction product.
The catalytic activity of the titanium trichloride thus obtained is not entirely satisfactory and the stereoregularity of the polymers produced is also not entirely satisfactory. Thus various improvements have been attempted.
Japanese Patent Publication No. 92,298/1973, for example, discloses that a titanium trichloride composition obtained by reducing titanium tetrachloride with an organoaluminum compound and heat-treating the reduction product can be improved by pulverizing the titanium trichloride composition in a ball mill together with other compounds, and then extracting and washing the pulverized product with a solvent.
As improved methods for the preparation of a titanium trichloride composition represented by the formula TiCl.sub.3 (AlCl.sub.3).sub.1/3 hereinafter (TiCl.sub.3 AA) by reducing titanium tetrachloride with metallic aluminum and pulverizing the reduction product in a ball mill, Japanese Published Patent Application (OPI) Nos. 21777/1973, 3188/1975 and 83781/1974 disclose the ball-milling of TiCl.sub.3 AA together with other compounds, and Japanese Published Patent Application (OPI) No. 60182/1973 discloses a method for preparing the solid catalyst by ball-milling TiCl.sub.3 AA together with other compounds and then washing the pulverized product with an inert solvent.
All of these improved methods require the pulverization of the titanium trichloride composition in a ball mill, and still the catalysts obtained have insufficient catalytic activity.
It is also known to improve TiCl.sub.3 AA by treating the TiCl.sub.3 AA with an organoaluminum compound containing bromine or iodine and an alkyl halide, as disclosed in Die Makromoleculare Chemie, 176, p. 2159 (1975), and Catalyst, Vol. 18 (No. 2), page 2, 1976.
Japanese Published Patent Application (OPI) Nos. 81889/76 and 117787/76 suggest a method for improving the catalytic performance of TiCl.sub.3 AA, which comprises treating titanium trichloride with iodine or bromine, or a hydrocarbon compound of iodine or bromine. This method produces some improvement in the stereoregularity of the polymers produced therewith, but substantially no improvement in the catalytic activity.
Further, Japanese Published Patent Application (OPI) No. 143,790/1975 discloses treatment of the solid resulting from the reduction of titanium tetrachloride with aluminum powder or an organoaluminum compound with a mixture of a complexing agent and carbon tetrachloride. The disclosure is that after drop-wise adding an organoaluminum compound to titanium tetrachloride at a temperature of -10.degree. C. to 10.degree. C., the reaction may be completed by increasing the temperature of the reaction mixture to 20.degree. to 100.degree. C., but preferably the reaction is completed by maintaining a low temperature without increasing the temperature. Since, however, the solid catalyst thus obtained is very unsatisfactory in terms of catalytic activity and with respect to the stereoregularity of the polymers formed, it is desirable to repeat the carbon tetrachloride treatment once more.